Cobalt(I) Pincer Complexes as Catalysts for CO2 Hydrogenation to Formate

Carbon dioxide hydrogenation with a base to generate formate salts can provide a means of storing hydrogen in an energy-dense solid. However, this application requires catalytic CO₂ hydrogenation, which would ideally use an earth-abundant metal catalyst. In this article, six new (CNC)Co^IL₂ pincer complexes were synthesized and fully characterized including single-crystal X-ray diffraction analysis on four new complexes. These complexes contain an imidazole-based (1_R) N-heterocyclic carbene (NHC) ring or a benzimidazole-based NHC ring (2_R) in the CNC pincer. The R group is para to N on the pyridine ring and has been varied from electron-withdrawing (CF₃) to donating (Me, OMe) substituents. The L-type ligands have included CO and phosphine ligands (in ^PPh32 and ^PMe32). Thus, two known Co complexes (1, 1_OMe) and six new complexes (1_Me, 1_CF3, 2, 2_OMe, ^PPh32, and ^PMe32) were studied for the CO₂ hydrogenation reaction. In general, the unsubstituted CNC pincer complexes bearing two carbonyl ligands led to the highest activity. The best catalyst, 2, remains active for over 16 h and produces a turnover number of 39,800 with 20 bar of 1:1 CO₂/H₂ mixture at 60 °C. A computational study of the mechanism of CO₂ hydrogenation is also reported.